310 THE BIOLOGY OF HYDRA : 1961 



tion, it is clear that distal-most tissues have held rigidly to the se- 

 quence of events characteristic of these tissues in normal 

 development. 



CONCLUSIONS 



It is clear that the pattern of colonial growth can be altered in 

 Campanularia by changes both in temperature and in nutritive 

 level. The alterations are largely due to the sensitivity of zones 

 of prospective growth. 



Differences in hydranth morphogenesis are striking when one 

 compares the processes in thecate and athecate species. In the 

 thecate form, Campanularia, a hydranth of full size is produced by a 

 series of events which are not easily altered; they show little abil- 

 ity to regulate. Once produced thecate hydranths do not grow, 

 they do not regenerate parts which have been removed, and they 

 regress and are resorbed after living for only a few days. In all 

 these respects the reverse is true for athecate species. 



We know that old parts are utilized for new growth, but we do 

 not know in what form materials are moved: as tissues? cells? 

 fragments? chemical substances? This needs study. More attention 

 also should be given to the initiation of new growth by zones of 

 prospective growth. For analysis of these particular problems the- 

 cate species, such as Campanularia, are probably better than athe- 

 cate forms. 



ACKNOWLEDGEMENTS 



The author must acknowledge the assistance of Malcolm Rusk 

 and Ruth Curtiss Telfer who were with him at the beginning of 

 the studies of Campanularia; of Charles Wyttenbach who has made 

 many contributions of ideas and time and whose photographs have 

 been copied here; of Fred Wilt, Richard Manassa, Annelle Gibbon, 

 Jean Lowiy, Maurice Hartman, and Pat Clapp all of whom have 

 had some part in the work summarized here. 



The paper ought to be dedicated to the memory of Frederick 

 S. Hammett who long ago proclaimed the special virtues of Cam- 

 panularia for studies of growth. 



